The use of gait analysis in clinical settings has become widespread. Evaluation of a patient's walking pattern requires the attachment of optical targets to the skin's surface. Data derived from monitoring the location of these targets while the patient walks provide the basis for objective human movement studies. In evaluating these data, the assumption that target positions indicate skeletal position is routinely ignored. This study's purpose is to evaluate the magnitude of error produced by tracking the skeletal system using skin-mounted targets. To date, the gait's of four subjects have been tested using surface- and bone-mounted targets affixed to the shank. The results indicate that kinematic errors occur primarily about and along the long axis of the lower leg. It does not appear feasible to model these errors taking into account only the passive motion of soft tissues. Quantification of the errors associated with standard gait analysis procedures is important in the test validation process. During the past year, we failed in several attempts to gain rigid fixation on the targets to the femur while maintaining an acceptable level of subject comfort and knee joint motion. We have abandoned further attempts at femoral fixation and terminated the protocol in July 1996.